Magnetic resonance imaging (MRI) is a technique used frequently in medical settings to produce images of the inside of the human body. MRI is based on detecting nuclear magnetic resonance (NMR) signals emitted by molecules under the influence of electro-magnetic fields. In particular, magnetic resonance (MR) techniques involve detecting electro-magnetic changes resulting from re-alignment of atomic spin of molecules in the tissue of the human body. MR techniques may be used to study fluid flow, such as, for example, blood flow and/or blood perfusion in tissue. One of many possible applications is the study of blood perfusion in the human brain.
During an MRI procedure, NMR signals emitted from a volume of interest or from a slice (i.e., a relatively thin region) of the volume of interest are detected and/or otherwise obtained. The acquired NMR signals may then be reconstructed to form a two dimensional (2D) image of the slice. A plurality of such 2D images reconstructed from NMR signal data obtained from successive slices may be stacked together to form a three dimensional (3D) image. A 2D image is comprised of pixels, each pixel having an intensity (e.g., a magnitude or value) that is proportional to the strength of the NMR signal emitted by a corresponding location in the volume of interest. Similarly, a 3D image is composed of voxels, each voxel having an intensity proportional to the strength of the NMR signal emitted from a corresponding portion of the volume of interest.
As discussed above, MRI exploits the NMR phenomenon to distinguish various tissue characteristics. In particular, MRI operates by manipulating spin characteristics of tissue, and more specifically, hydrogen atoms of water molecules which compose a significant proportion of the human body, including both blood and tissue. MRI techniques include aligning the spin characteristics of hydrogen nuclei in a magnetic field, and perturbing the magnetic field with radio frequency (RF) signals.
The NMR phenomenon is invoked by the RF signals, applied at the Larmor frequency, exciting the hydrogen nuclei and causing the spin to briefly precess about an axis in the direction of the applied RF signal, rather than in the direction of the applied magnetic field. The Larmor frequency is related to the rate at which a nucleus precesses about an axis, which is, in turn, proportional to the strength of the applied magnetic field. When the RF signal subsides, the spins gradually realign with the magnetic field, releasing energy in the process. The released energy may be detected and used to form one or more images representative of the hydrogen content of the tissue. The NMR signals may be detected using one or more RF coils sensitive to electromagnetic changes caused by the NMR signals. The RF coils may be the same or different than RF coils, that when driven by a signal generator, provide the RF signals used to invoke the NMR phenomenon.
Using these fundamental principles, fluid content may be measured in a variety of substances or tissue, by measuring characteristics of the tissue's NMR response. In order to detect fluid flow or perfusion in a particular region of interest, fluid flowing into that region may be “labeled” by reversing, or perturbing, the spins of the protons of the fluid in some region that is “upstream” from the region of interest, and then detecting the labeled fluid when it flows through or is perfused into the region of interest. Although terms “flow” and “perfusion” may sometimes be used interchangeably, perfusion as used herein refers to a diffusible exchange between a fluid and a substance, such as, for example, human tissue. The term “flow” as used herein, generally refers to flow of liquid in vessels, such as, for example, flow of blood in arteries. The term “labeling” refers herein to preparing atomic spins such that, upon relaxation or recovery, a detectable NMR signal is produced.
One strategy for spin labeling includes providing RF signals that result in spin inversion for atoms exposed to the RF energy. The inversion recovery (i.e., the process of the atoms returning from the induced inverted spins) emits an NMR signal that can be detected to measure blood flow and/or perfusion. Spin inversion may be achieved by generally aligning the spins in a magnetic field, and inverting the spins by applying an RF field, typically, in a direction orthogonal to the magnetic field, as discussed above. A number of RF field waveforms, referred to herein as an RF sequence, that achieve spin inversion are generally known. However, conventional RF sequences have several drawbacks, as discussed in further detail below.
By applying a gradient magnetic field to align the spins, the spin inversion effect may be localized to a particular region of interest. In particular, to achieve spin inversion, the RF field is applied at an appropriate frequency (i.e., the Larmor frequency), which depends, at least in part, on the strength of the magnetic field. Thus, an RF field applied at a particular frequency will only induce spin inversion at portions of the gradient magnetic field where the RF frequency matches the Larmor frequency at the corresponding magnetic field strength. By appropriately selecting the gradient magnetic field and RF frequency, spin inversion effects may be spatially isolated such that only spins in a region of interest are labeled.
However, despite localization efforts, magnetization transfer effects and other unrelated errors may interfere with the labeling procedure by causing more than just the atoms in the region of interest to be labeled, which in turn results in artifacts in the reconstructed images. In order to account for such effects, a control procedure may be used wherein the magnetic field gradient and RF sequence are selected to mimic the unrelated effects without invoking spin inversion. MR images reconstructed from NMR data obtained after the labeling procedure and control procedure may be used to reduce or eliminate these unwanted effects, for example, by subtracting out the effects associated with one or more control images to remove at least some of the image artifacts from the labeling images.
MR techniques in general endeavor to achieve a balance between signal to noise ratio (SNR) and power deposition. In particular, the higher the energy of the magnetic fields used (and correspondingly the higher the energy of the RF sequences needed to invoke the NMR effect), the greater the SNR of the NMR signals. Accordingly, higher energy MR results in higher contrast, better quality images. However, performing MRI at higher energies results in increased RF power deposition. There are limits to the RF power that may be deposited in the human body without harming the tissue.